Rare earth permanent magnets and their preparation

Abstract

A sintered magnet body (RaT1bMcBd) coated with a powder mixture of an intermetallic compound (R1iM1j, R1xT2yM1z, R1iM1jHk), alloy (M1dM2e) or metal (M1) powder and a rare earth (R2) oxide is diffusion treated. The R2 oxide is partially reduced during the diffusion treatment, so a significant amount of R2 can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries. The coercive force is increased while minimizing a decline of remanence.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is a Divisional of copending U.S. patent application Ser. No. 13 / 461,043 filed on May 1, 2012, which is a non-provisional application which claims priority under 35 U.S.C. §119(a) on Japanese Patent Application Nos. 2011-102787 and 2011-102789 filed in Japan on May 2, 2011 and May 2, 2011, respectively, the entire contents of which are hereby incorporated by reference.TECHNICAL FIELD[0002]This invention relates to an R—Fe—B permanent magnet having an enhanced coercive force with a minimal decline of remanence, and a method for preparing the same by coating a sintered magnet body with a mixture of an intermetallic compound, alloy or metal powder and a rare earth oxide and heat treating the coated body for diffusion.BACKGROUND ART[0003]By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the applicati...

AI Technical Summary

Benefits of technology

The patent describes a method of making rare earth magnets by coating them with a mixture of rare earth oxide and intermetallic compound or metal powder. This results in the rare earth element being introduced near the interfaces of the main magnet grains, which increases the coercive force while minimizing the decline of remanence. This process also increases productivity and results in a magnet with excellent magnetic performance. Despite using minimal amounts of rare earth elements, the magnet exhibits increased coercive force and minimal decline of remanence.

Problems solved by technology

The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators.

When Dy or Tb is added in an ordinary way, however, a loss of remanence is unavoidable because Dy or Tb substitution occurs not only near the interface of the primary phase, but even in the interior of the primary phase.

Another problem arises in that amounts of expensive Tb and Dy must be used.

The results are still not fully satisfactory.

However, the processes utilizing evaporation or sputtering have many problems associated with units and steps when practiced on a mass scale and suffer from poor productivity.

However, since Dy or Tb diffuses through substitution reaction between the powder and the magnet component, it is difficult to introduce a substantial amount of Dy or Tb into the magnet.

The method is advantageous for introducing a substantial amount of Dy or Tb into the magnet, but less productive because the calcium or calcium hydride powder needs careful handling.

The method of coating with only metal alloy has the drawback that it is difficult to coat the metal alloy onto the magnet surface in a large and uniform coating weight.

However, this method requires many steps as compared with the method of coating a mother alloy magnet with a dispersion of a powder mixture of an intermetallic compound and a rare earth oxide in a solvent.

The method is time consuming and is not industrially useful.